AsNoriuanswered grow question 2y ago Autos dont care about how long day or night is. So basically you can change anytime, BUT in rare cases plant can go a bit stunt or even start to hermie.
Yours are far in flower, so all should be good.
Better use 18/6 with autos, 24/0 is too much for plants, they need rest.
Take off all dead leaves from bottom of pot, not wise idea to leave them there for pests or mold to have great conditions.
Take out that room plant too, it looks sick and can bring diseases to girls too.
On one photo it looks like plant is sprayed. Never spray canna plants in flower !
Good luck in finishing them !